Wastepaper is an abundant source of wood fibre which, if recycled, can be a lower-cost alternative to wood pulp in a variety of paper products. Before wastepaper can be reused, however, it is necessary to subject the wastepaper to a deinking procedure to remove inks and coating materials from the wood fibre.
The most commonly employed deinking process is a flotation process in which the wastepaper is washed and treated with dilute sodium hydroxide in a reaction tank, causing the fibres to swell and loosening up the ink and coating material particles, such as clay, calcium carbonate, titanium dioxide and talc. Peroxides and surfactant next are added to bleach and whiten the fibres and to disperse the ink and coating material particles and render them hydrophobic, so that they can attach to rising air bubbles passing through the tank. This action separates the ink particles from the fibres and forms a foam containing ink particles on the surface of the tank.
The foam then is skimmed off the pulp dispersion. The removed ink particles, together with coating material particles, rejects, residual fibres and water, form a slurry of about 3 to 5 wt % solids. This slurry is filtered and dewatered to provide a residue known as deinking sludge, with an about 25 to 55 wt % solids content. This material is referred to herein as deinking by-product (DBP). The solids content of this by-product generally comprises about 60 to 80 wt % organic material, mainly wood fibres, and the balance ash, i.e. coating material particles. The amount and composition of the deinking by-product varies widely with the type of wastepaper recycled and the efficiency of the deinking process employed. For a typical deinking plant of 250 tons/day recycle fibre production capacity, approximately 70 tons/day of deinking by-product is produced.
As the interest in recycling wastepaper has increased greatly in recent years, the disposal of large quantities of deinking by-product has become an important environmental issue. However, none of the three current major means of disposal is particularly satisfactory. Landfilling may become environmentally abusive in the long term. Such landfill sites can be costly since they are required to have liners, leachate collection systems and ground water monitoring systems.
Soil conditioning is an attractive means of disposal, in view of the high organic content of the sludge. However, there remains an uncertainty over the effects of minor components in the deinking by-product on the quality of produce grown on the treated soil, and concern over possible associated health risks.
Incineration is an attractive alternative since landfill requirements for ash disposal are significantly diminished. However, the environmental effects of the gaseous products of incineration are uncertain as is the disposal of the ash.
Gypsum is a hydrous calcium sulphate, having the formula CaSO.sub.4.2H.sub.2 O. Calcined gypsum, i.e. the hemihydrate form, is variously known under the terms plaster of Paris, stucco, calcined plaster and plaster and has the property of hardening or setting upon rehydration. This property is put to a variety of plastering uses and is used to produce a variety of commercial products, including building materials, particularly drywall or wallboard. The term stucco is used herein to refer to calcium sulfate hemihydrate, although any of the other common terms used to describe calcium sulfate hemihydrate may be employed.
A typical drywall manufacturing process involves sandwiching a stucco composition between two paper-board sheets on a continuous ribbon machine. The front surface paperboard is fed continuously with the eventual front surface face-down on a rubber conveyor belt. The edges of this paper sheet are curled up to provide a trough into which a stucco slurry may be poured. The slurry is mixed continuously from stucco powder and water, with a foaming agent usually being added to provide a lower density product. Other ingredients may be added to modify the properties of the slurry.
The mixer in which the slurry is formed also generally is a pump which pumps the slurry onto the forming machine. A portion of the slurry usually is directed from the mixing pump to a high intensity mixer which beats out the entrained air. This higher density material is directed to the ribbon edge to provide a stronger, increased density edge for edge handling protection.
The board backing paper is fed onto the top of the lower ribbon and the board is assembled by passing through a set of rolls which form the board by rolling over the edge of the front paper, placing the backing paper on top and determining the board thickness. A strip of glue is rolled over the rolled-over front paper edge to which the backing paper is adhered.
The board formed in this way is still fluid and needs to be passed through sizing guides to maintain the desired dimensions. As the board passes along the conveyor belt, the plaster gradually sets. Because of density differences within the plaster and because the bottom and top papers relax due to wetting at different rates, the edge and back of the board usually are sprayed with water to prevent the board from rippling or buckling.
The speed of conveying and length of conveyor belt depend on the set time required. For example, the continuous ribbon may travel at about 300 ft/min for about 600 feet, at which point it can be transferred to rollers from the conveyor belt since it is sufficiently set and does not easily deform. The ribbon may travel for about a further 400 feet on the rollers, giving a total set time of 3 to 4 minutes, prior to cutting the ribbon into individual sheets. If chamfered edge board, used for taped joint applications, is required, a separate plastic edge mould strip may be fed along the conveyor belts with the ribbon.
The ribbon is sheared into lengths from 14 to 24 feet and flipped over so that the finished surface is upwards. This arrangement prevents the surface from being marred by rollers in the drying oven. The sheets may be stacked side-by-side or in-line. The groups are fed to a multi-desk drying oven in sequence and removed in the same sequence at the end of the desired drying time. The sheets travel over driven rollers in the ovens and are subjected to hot combustion gases, for example, at about 630.degree. F. to dry the sheets in about 30 minutes residence time. The interior of the sheet is kept cool by water evaporation which prevents the dehydration of the newly-formed gypsum. Sheets exit the drier at about 180.degree. to 200.degree. F. The sheets are restacked face to face and cut to the required lengths for shipping.